Thin films of amorphous silicon (abbreviated to a-Si) are important for solar cells and many other photoelectronic devices. Usually thin films of a-Si are formed by a chemical vapor deposition (CVD) method, and currently it is prevailing to employ a plasma CVD method which uses glow discharge to produce a non-equilibrium and reactive plasma by decomposing a raw material gas such as silane gas. The plasma CVD method is advantageous in that a film of a-Si can be deposited on a substrate at a relatively low temperature and that a-Si films of fairly large areas can easily be obtained.
With respect to a-Si films formed by the conventional CVD methods, a serious problem is that the photoelectronic properties of the films are considerably degraded by irradiation with light. The degradation is known as the Staebler-Wronski effect and is represented by a considerable decrease in the photoconductivity. For example, by irradiation with light the photoconductivity of an intrinsic a-Si film decreases to the extent of about 1/10 of the initial value.
Much efforts have been directed to a solution of the photo-induced degradation problem, but a really satisfactory result has not been obtained yet. For example, JP 59-54274 A proposes to introduce a small amount of an element of the Group V, such as nitrogen, into an a-Si film deposited by the plasma CVD method by mixing a hydride gas such as NH.sub.3 gas with the silane gas. JP 63-84079 A relates to a hydrogenated amorphous silicon film and proposes to regulate the CVD operation conditions such that in the hydrogenated amorphous silicon only less than about 1% of silicon atoms bond to two hydrogen atoms, respectively. JP 2-219284 A proposes to mix helium gas with silane gas in the proportion of, for example, about 5:1 by volume. However, these measures are not yet sufficiently effective for suppression of the photodegradation of the obtained a-Si films.
Meanwhile, in respect of the rate of growth of an a-Si film in the plasma CVD method, JP 1-294866 A proposes to mix silane gas with up to 2 vol % of xenon gas for the purpose of promoting the decomposition and activation of silane gas and thereby enhancing the film growth rate without degrading the properties of the obtained film.
Also it is known to produce a-Si films by a photo-initiated CVD method with a view to obviating a problem in the plasma CVD method that the properties of the obtained films may be adversely affected by mobile ions and radiation. However, the photo-initiated CVD method also has some inherent disadvantages, and at present this method is not favorable for practical manufacture of a-Si films.